In many applications of filtration technology, it is highly desirable to utilize a porous membrane which is both hydrophilic and has desirable bulk properties, such as mechanical strength, flexibility, thermal stability, and chemical stability. Since many known polymers having desirable bulk properties are hydrophobic, it has become important to develop membranes which have the desirable bulk properties of hydrophobic polymers, yet have desirable hydrophilic surface properties which differ dramatically from the properties of the hydrophobic support polymer. Desirable surface properties include wettability, low protein adsorption, thromboresistance, controlled ion exchange capacity, and controlled surface chemical reactivity.
One method of preparing hydrophilic membranes involves graft polymerizing a hydrophilic monomer onto the surface of a porous hydrophobic polymeric membrane substrate. However, it is difficult to apply a graft coating to the entire surface of the substrate without clogging the pores thereof. A typical example of a photochemical grafting process used to hydrophilically modify the surface of a hydrophobic polymer is described in U.S. Pat. No. 5,468,390. A method for grafting a non-graftable substrate is described in U.S. Pat. No. 5,547,575. That method involves coating the substrate with an N-containing polymer, halogenating the coating to form an intermediate N-halogenated species, and then grafting the treated substrate with an ethylenically unsaturated hydrophilic monomer. While this approach might be useful, it is expensive, requiring a coating as well as an N-halogenated intermediate, which is eventually displaced upon grafting of the hydrophilic monomer.
Another method that can be used to impart hydrophilic surface properties to a hydrophobic substrate is to coat a preformed hydrophobic membrane with surfactants or water-soluble polymers, such as polyvinyl pyrrolidone. This approach is substantially limited, particularly due to flux reduction caused by reduction of pore size. Flux reduction is especially severe for membranes having small pore dimensions (e.g., &lt;0.1 .mu.m).
An attempt to address the problem of pore clogging can be found in U.S. Pat. No. 4,976,897 (the '897 patent) which discloses a microporous support coated with a UV curable resin having a sufficiently high viscosity to allegedly prevent pore filling upon coating and curing. In order to diminish pore clogging, the '897 patent requires the coating resin to have a viscosity of at least 35,000 centipoise, preferably 50,000 to 500,000 centipoise. As a consequence, this approach is quite limited in terms of the materials which can be used to coat the surface of the membrane. A resin having a low viscosity will result in the resin pooling into the interiors of the pores during the curing process, thereby causing pore clogging and restricted flow across the membrane. Further, resins having viscosities in the higher range may be inappropriate depending on the application. If the porous polymeric support has too much variation with respect to individual pore diameters, the coating will be unevenly distributed, clogging some pores and insufficiently coating the surfaces of other pores. More importantly, even if the resin viscosity is balanced to maximize membrane coating while minimizing pore blockage, the coating typically will avoid some pore interiors. Thus, only the exterior surface of the membrane will be rendered hydrophilic. Since the porous interior constitutes the vast majority of the surface area of a porous membrane, the hydrophilic modification in the '897 patent leaves a significant amount of the surface area of the membrane (i.e., the pore interiors) hydrophobic.
U.S. Pat. No. 4,618,533 (the '533 patent) describes a porous hydrophobic membrane which is modified with a crosslinked hydrophilic polymer such that the entire surface purportedly is rendered hydrophilic without clogging the pores. The membrane is prepared by soaking a porous hydrophobic polyvinylidene floride (PVDF) membrane in a reagent bath containing a free radical polymerizable monomer (e.g., hydroxypropyl acrylate), a polymerization initiator, and a crosslinking agent (e.g., tetraethyleneglycol diacrylate) such that the reagent bath wets the entire surface of the porous membrane. The polymerization and crosslinking reaction is then carried out using high temperatures (e.g., 95.degree. C.). U.S. Pat. No. 4,917,793 applies the method of the '533 patent toward a polytetrafluoroethylene (PTFE) membrane to produce membranes which are transparent in aqueous media.
U.S. Pat. No. 5,158,721 (the '721 patent) purportedly discloses a hydrophilic membrane formed from an interpenetrating network of a hydrophobic polymer and a hydrophilic crosslinked monomeric composition. The membrane is prepared by casting a thin film of the precursors on a substrate and exposing the film to low intensity UV light to form the interpenetrating polymeric network. The polymeric network is then coagulated to form a porous membrane. The membrane then must be annealed in order to render the surface of the resulting membrane hydrophilic. The annealing causes the hydrophobic polymer to become more crystalline thereby excluding the hydrophilic polymer from the crystalline regions and rendering the membrane surface hydrophilic. Although this approach avoids some of the aforementioned disadvantages of thermal coating, it nevertheless involves heat-treating the membrane at high temperature to induce crystallization of the bulk polymer. Further, the '721 patent teaches that the hydrophobic polymers employed in the disclosed process must be crystalline polymers in order to render the membrane surface hydrophilic after annealing. The requirement of a crystalline bulk polymer is disadvantageous in that it precludes the use of a large number of non-crystalline bulk polymers with excellent properties (e.g., mechanical strength, chemical resistance, and oxidative stability).
Thus, there remains a need for a porous hydrophilic membrane having the desirable bulk properties commonly found in hydrophobic polymers and also having hydrophilic surface properties uniformly distributed throughout the entire surface of the membrane. Further, there is a need for a hydrophilic membrane which is "hydrophilically durable" in that it retains its hydrophilicity under conditions of extended and/or frequent contact with a hydrophilic fluid. Moreover, there is a need for an efficient method to prepare such membranes. The present invention provides such a porous hydrophilic membrane and method of preparing same. These and other objects of the present invention will be apparent from the detailed description of the preferred embodiments of the invention set forth hereinbelow.